The shell surface of the types of press rolls discussed above consists of a perforated metal sheet, which is welded onto a roll body. In order to achieve a high capacity, the total open area must be large, and at the same time the apertures must be sufficiently small so as to prevent fibers from following along with the liquid through the apertures. In order to solve these problems, the press roll can be covered with a wire cloth on the outer surface of the perforated shell sheet. Such a wire permits liquid flow even along the shell surface. This, in turn, permits the perforations to be given a greater size, whereby the capacity is increased without risking increased fiber passage. Wire-covered press rolls have proven to be particularly advantageous for mechanical pulp, which contains a large amount of fine material. Compared with a perforated metal sheet, the wire cloth permits a smaller amount of fine material to pass through the apertures.
The rolls are generally rotatably mounted on two parallel shafts, and are usually immersed into a trough, which encloses at least the lower half of the rolls. The material suspension is supplied to the space between the rolls and the trough. This space is sealed from the surroundings so that an overpressure can be maintained in the trough whereby the dewatering is facilitated. The pulp is moved from the trough upwards through the nip, where it is simultaneously dewatered. Doctor blades are located after the nip, for the purpose of removing the dewatered pulp web from the rolls. The pulp is then removed by a conveyor for transfer to subsequent processing steps.
By means of this type of dewatering, dry matter contents of up to about 55 to 60% can be obtained. The equipment required for removing the dewatered pulp web from the rolls and for transferring the pulp from below to the conveyor, however, is complicated and sensitive to operational breakdowns. In many cases when a maximum dry matter content is not required, therefore, a simpler dewatering apparatus would be sufficient and quite desirable.